MORGAN HILL, Calif. —- A 59-year-old preschool teacher has been arrested on child endangerment charges for allegedly putting an unknown substance into the drinking cups of her young students.

Morgan Hill police said Deborah Gratz, of Hollister, had been arrested after Kiddie Academy preschool officials reported that she had put Sominex -- an over the counter sleeping aid -- into the drinking cups of some of the children on Friday March 8, 2013.

A witness told authorities they observed Gratz placing an unknown substance into the drinking cups in her classroom filled with toddlers between one and two years of age.

Investigators said they were still working on a motive.

Gratz, a five-year veteran employee of the facility, was arrested after a search of her home on Monday turned up evidence in the case.

She was placed under arrest on felony charges of child endangerment and booked in the Santa Clara County main jail.